1. Field of the Invention
The present invention relates generally to two-way messaging devices or pagers (hereafter "pagers"), and particularly to a pager whose transmitter is powered only when the pager is in range of a base station receiver.
2. Discussion of the Known Art
Known pager systems use at least one base station for communicating with a number of portable pagers. Pagers and other user equipment assigned to the system are generally referred to as subscriber units (SU's). Over a forward or "down-link" frequency channel, a base station transmits down-link signals containing information destined to individual SU's whose addresses are also encoded in the down-link signals. If so configured, a two-way pager can transmit an acknowledgment signal back to the base station (an "ack-back" signal) to confirm its detection of a down-link signal addressed to it. In addition, the pager may be capable of transmitting other "up-link" signals to a base station, for example, to provide information that allows the pager's location to be established.
It is contemplated that a wireless communication system capable of global coverage for all subscribers, will soon be available. The system, known as "IRIDIUM", is presently specified to include mobile telephony and one-way pager service world-wide using a number of communication satellites in low-earth orbit (LEO). A satellite-based pager service will overcome certain limitations of terrestrial based systems. At present, people traveling with pagers know that paging systems around the world are not usually compatible. The United States (900 MHZ FLEX), South Korea (322 MHZ FLEX), and France (VHF ERMES), all have different paging frequencies and protocols. While several paging service providers have achieved regional and even some international coverage, such services still exclude major portions of the world. The result is that paging subscribers have few options for roaming worldwide and, sometimes, even on a regional basis.
By contrast, a mobile satellite system may operate according to a uniform protocol worldwide. Because a system subscriber may want to receive pages at any time while traveling globally, it is important that the system be capable of determining the subscriber's approximate location on the earth, so that pages destined to the subscriber can be transmitted from a base station (satellite) closest to the subscriber. Thus, a "two-way" or acknowledgment type of pager unit is desirable for use in paging systems extending over large geographic areas, and particularly for use in a global paging system.
A pager may not always be within range of a base station to detect a message sent from the base station. For example, the pager may be inside a building that attenuates signals from the base station below a threshold value needed for accurate pager reception. So-called received signal strength (RSS) circuits have been provided for terrestrial-based paging systems, to alert users when their pagers are out of range from a base station transmitter and that messages transmitted from the base station may not have been detected by their pagers. See, for example, U.S. Pat. No. 5,032,835 (July 1991); U.S. Pat. No. 4,851,820 (July 1989); U.S. Pat. No. 5,049,875 (September 1991); and U.S. Pat. No. 5,289,178 (Feb 1994). All relevant portions of these patents are incorporated by reference herein. Again, with a two-way pager system, if an acknowledgment or location signal from a system pager is not detected by a base station receiver, the system can re-transmit a message periodically until the addressed pager's signal is detected.
Even if a two-way pager receives and accurately detects a message signal addressed to it, power limitations of the pager's transmitter could make it unlikely that a signal radiated from the pager will be detected at a base station. Since it is desirable to use small and readily available batteries, e.g., size "AA" or "AAA" to power a pager, its transmitter radio frequency (RF) power output usually does not exceed 1000 milliwatts. Pager signal field strength is also limited by the size of the pager antenna, which for ease of portability should not exceed the size of the pager itself.
Thus, while base station transmitters can produce strong signals at RF power levels of, e.g., 150 watts allowing pagers located over wide geographic areas to detect them, signals radiated from the pagers have a lesser chance of being detected at a system base station. And, each time a pager transmits a signal, substantial battery power is consumed whether or not the signal is detected adequately at a base station. It would therefore be desirable to avoid wasting pager battery power when it is unlikely that a signal to be radiated from a two-way pager would be detected by a base station. Such a restraint on two-way pager transmitter activation would allow for a lower pager transmitter RF power level, a smaller pager antenna, and a smaller and less expensive overall construction for the pager including its battery components.
Also, if the user is alerted whenever his or her pager's transmitter is unable to signal a base station, he or she can simply move to a more favorable location where the alert ceases. Such an alert feature would also allow the pager transmitter to require only the minimum amount of power to signal a system base station under favorable propagation conditions. That is, the pager transmitter is activated only under a "go" condition when there is a good likelihood that its signal will be detected by a base station, and the transmitter does not waste battery power in a "no go" situation when it is unlikely that the pager's signal will be received.